As a fast Algorithm, the P1-approximation method has been widely applied to the inversion of the radiative transfer equation. In order to study the solving precision of the P1-approximation in the graded index medium, two cases of radiative heat transfer in two-dimensional rectangular gray semitransparent medium enclosed by opaque and diffuse boundary are considered. The P1-approximation method is extended to solve the radiative transfer problem in a c semitransparent graded index and even medium, the temperature field and radiative heat flux are compared with the results obtained by the curved Monte Carlo method in references. The results show that the optical depth and the refractivity gradient of the medium have great influence on the solving precision of the P1-approximation. The solving precision of the P1-approximation will be worse because of the uneven heat flux distribution induced by the curved ray trajectories in the graded index medium, but the solving precision will be revised under special circumstances for the anisotropic scattering media.
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